1. Field of the Invention
The present invention relates to a liquid crystal compound, a liquid crystal composition, and a liquid crystal display device. More specifically, the invention relates to a chlorofluorobenzene derivative, which is a liquid crystal compound, a liquid crystal composition containing the compound and having a nematic phase, and a liquid crystal display device containing the composition.
2. Related Art
A liquid crystal display device utilizes optical anisotropy and dielectric anisotropy inherent to a liquid crystal compound (which means, in the invention, a generic term for a compound having a liquid crystal phase such as a nematic phase, a smectic phase and so forth, and also for a compound having no liquid crystal phase but being useful as a component of a composition). On a liquid crystal display device, various modes including a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, and so forth.
Among these operation modes, an ECB mode, an ISP mode, a VA mode and so forth are operation modes utilizing vertical orientation of liquid crystal molecules, and it has been known that in particular an IPS mode and a VA mode are capable of improving the narrow viewing angle, which is a problem of the conventional display modes, such as a TN mode, an STN mode and so forth.
As a component of a liquid crystal composition having a negative dielectric anisotropy capable of being used in a liquid crystal display device of these operation modes, a large number of such liquid crystal compounds have been investigated that hydrogen on a benzene ring is replaced by fluorine (see, for example, JP H2-503441 A/1990 and JP H02-4725 A/1990).
For example, a compound (A) represented by the following structural formula has been investigated (see, for example, JP H2-503441 A/1990). However, a compound, in which hydrogen on a benzene ring is replaced by fluorine, represented by the compound (A) is poor in compatibility with other compounds in a low temperature region.
As a compound, in which hydrogen on a benzene ring is substituted by chlorine and fluorine, for example, a compound (B) represented by the following structural formula has been reported (see, for example, WO 98/23561 A/1998). However, the compound (B) does not have a high clear point and has a large viscosity.
As a compound, in which hydrogen on a benzene ring is substituted by chlorine, for example, a compound (C) represented by the following structural formula has been reported (see, for example, Molecular Crystals and Liquid Crystals, vol. 260, pp. 227-240 (1995)). However, the compound (C) has a significantly narrow range (mesomorphic range) where the compound shows liquid crystallinity and has a small negative dielectric anisotropy.
As chiral dopants in liquid-crystalline media for electro-optical displays, for example, a compound (D) represented by the following structural formula has been reported (see, for example, JP H10/158,652 A/1998). However, the compound (D) has a significantly narrow range (mesomorphic range) where the compound shows liquid crystallinity.

Therefore, a liquid crystal display device having such an operation mode as an IPS mode, a VA mode and so forth still has problems in comparison to CRT, and improvement in response time, improvement in contrast, and decrease in driving voltage are demanded.
The liquid crystal display device driven in an IPS mode or a VA mode is constituted mainly by a liquid crystal composition having a negative dielectric anisotropy, and in order to improve the aforementioned characteristics, a liquid crystal compound contained in the liquid crystal composition is demanded to have the following characteristics (1) to (8).
Namely, the compound:
(1) is chemically stable and physically stable,
(2) has high clear point (transition temperature between a liquid crystal phase and an isotropic phase),
(3) has low minimum temperature of a liquid crystal phase (such as a nematic phase, a smectic phase and so forth), particularly a low minimum temperature of a nematic phase,
(4) has small viscosity,
(5) has suitable optical anisotropy,
(6) has suitable negative dielectric anisotropy,
(7) has suitable elastic constants K33 and K11 (K33: bend elastic constant, K11: splay elastic constant), and
(8) is excellent in compatibility with other compounds.
In the case where a composition containing a chemically and physically stable compound as in (1) is used in a display device, a voltage holding ratio can be increased.
In the case of a composition containing a liquid crystal compound having a high clear point or a low minimum temperature of a liquid crystal phase as in (2) or (3), the temperature range of a nematic phase can be enhanced, whereby the display device can be used in a wide temperature range.
In the case where a composition containing a compound having small viscosity as in (4) or a compound having suitable elastic constants K33 and K11 as in (7) is used in a display device, the response time can be improved. In the case where a composition containing a compound having suitable optical anisotropy as in (5) is used in a display device, the display device can be improved in contrast.
A liquid crystal composition containing a liquid crystal compound having a negatively large dielectric anisotropy has low threshold voltage, and therefore, in the case of a display device using a composition containing a compound having suitable negative dielectric anisotropy as in (6), the display device can have a low driving voltage and small electric power consumption. In the case where a composition containing a compound having suitable elastic constant K33 as in (7) is used in a display device, the driving voltage of the display device can be controlled, and the electric power consumption thereof can also be controlled.
A liquid crystal compound is generally used as a composition obtained by mixing with other many liquid crystal compounds for exhibiting such characteristics that are difficult to obtain with a sole compound. Therefore, a liquid crystal compound used in a display device desirably has good compatibility with other compounds as in (8). A display device is used in a wide temperature range including temperatures below freezing point in some cases, and therefore, a compound used in the composition desirably has good compatibility in a low temperature range in some cases.